BMI is a ratio based on body weight and height. The obtained value is compared to the established standards; however, trends or changes in values over time are considered more useful than isolated or one-time measurements. BMI is highly correlated with body fat, but increased lean body mass or a large body frame can also increase the BMI.
Individuals who have a BMI below 24 (or who are 80% or less of their desirable body weight for height) are at increased risk for problems associated with poor nutritional status. In addition, a low BMI is associated with higher mortality rates in hospitalized patients and community-dwelling elderly (Landi et al., 2000; Landi, Zuccala, Gambassi et al., 1999).
Those who have a BMI of 25 to 29 are considered overweight; those with a BMI of 30 to 39 are considered obese; above 40 is considered extreme obesity (National Institutes of Health, 2000).
It is important to assess for usual body weight and height. Current weight does not provide information about recent changes in weight; therefore, the patient is asked about his or her usual body weight. Decreased height may be due to osteoporosis, an important problem related to nutrition, especially in postmenopausal women. A loss of 2 or 3 inches of height may indicate osteoporosis.
In addition to the calculation of BMI, waist circumference measurement is particularly useful for patients who are categorized as of normal weight or overweight. To measure waist circumference, a tape measure is placed in a horizontal plane around the abdomen at the level of the iliac crest. Men who have waist circumferences greater than 40 inches
and women who have waist circumferences greater than 35 inches have excess abdominal fat. Those with a high waist circumference are at increased risk of diabetes, dyslipidemias, hypertension, and cardiovascular disease (National Institutes of Health, 2000).